| core | K. Li |
| Internet-Draft | B. Greevenbosch |
| Intended status: Standards Track | Huawei Technologies |
| Expires: February 8, 2015 | August 7, 2014 |
Representing CoRE Link Collections in CBOR
draft-li-core-links-cbor-00
Web Linking (RFC5988) provides a way to represent links between Web resources as well as the relations expressed by them and attributes of such a link. In constrained networks, a collection of Web links can be exchanged in the CoRE link format (RFC6690).
This specification defines a common format for representing Web links in CBOR format (RFC7049).
Discussion and suggestions for improvement are requested, and should be sent to core@ietf.org.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 8, 2015.
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Web Linking [RFC5988] provides a way to represent links between Web resources as well as the relations expressed by them and attributes of such a link. In constrained networks, a collection of Web links can be exchanged in the CoRE link format [RFC6690] to enable resource discovery, for instance by using the CoAP protocol [RFC7252]. [I-D.ietf-core-links-json] defines a common format for representing Web links in JSON format.
The Concise Binary Object Representation (CBOR) [RFC7049] is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation.
When converting between CORE Link Format and CBOR, as usual, many samll decisions have to be made. If left without guidance, it is likely that a number of slightly incompatible dialects will emerge.
This specification defines a common format for representing Web Links in the CBOR format.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This section discusses the information model underlying the CORE Link Format payload.
An application/link-format document is a collection of web links ("link-value"), each of which is a collection of attributes ("link- param") applied to a "URI-Reference".
The URI-Reference is represented as a name/value pair with the name "href" and the URI-Reference as the value.
The link attributes are also represented as name/value pairs with attribute names and attribute values.
The information model of the CoRE Link Format can be summarized below:
+------------------+--------------------+
| name | value |
+------------------+--------------------+
| href | resource URI |
+------------------+--------------------+
| attribute name 1 | attribute value 1 |
+------------------+--------------------+
| attribute name 2 | attribute value 2 |
+------------------+--------------------+
| ... | ... |
+------------------+--------------------+
| attribute name n | attribute value n |
+------------------+--------------------+
Figure 1: CoRE Link Format Information Model
To reduce message size, it would be beneficial to encode "href" and the standardized attribute names specified in [RFC5988] and [RFC6690] as integers.
The encoding is summarized below:
+----------+---------------+
| name | encoded value |
+----------+---------------+
| href | 1 |
+----------+---------------+
| rel | 2 |
+----------+---------------+
| anchor | 3 |
+----------+---------------+
| rev | 4 |
+----------+---------------+
| hreflang | 5 |
+----------+---------------+
| media | 6 |
+----------+---------------+
| title | 7 |
+----------+---------------+
| type | 8 |
+----------+---------------+
| rt | 9 |
+----------+---------------+
| if | 10 |
+----------+---------------+
| sz | 11 |
+----------+---------------+
| ct | 12 |
+----------+---------------+
Figure 2: Link Attributes Encoding
The objective of the CBOR mapping defined in this document is to contain information of the formats specified in [RFC5988] and [RFC6690].
We straightforwardly map:
o the outer collection to an array of links (Major type 4)
o each link to a map of pairs of data items (Major type 5)
In the object representing a "link-value", each target attribute or other parameter ("link-param") is represented by a CBOR pair of data items.
The URI-Reference is represented as a name/value pair with the name "href" and the URI-Reference as the value.
</sensors>;ct=40;title="Sensor Index",
</sensors/temp>;rt="temperature-c";if="sensor",
</sensors/light>;rt="light-lux";if="sensor",
<http://www.example.com/sensors/t123>;anchor="/sensors/temp"
;rel="describedby",
</t>;anchor="/sensors/temp";rel="alternate"
Figure 3: Example from page 15 of [RFC6690]
becomes
85 # array(number of data items:5)
a3 # map(number of pairs of data items:3)
01 # unsigned integer(value:1, "href")
68 # text string(8 bytes)
2f73656e736f7273 # "/sensors"
0c # unsigned integer(value:12,"ct")
18 28 # unsigned integer(value:40)
07 # unsigned integer(value:7,"title")
6c # text string(12 bytes)
53656e736f7220496e646578 # "Sensor Index"
a3 # map(number of pairs of data items:3)
01 # unsigned integer(value:1,"href")
6d # text string(13 bytes)
2f73656e736f72732f74656d70 # "/sensors/temp"
09 # unsigned integer(value:9,"rt")
6d # text string(13 bytes)
74656d70657261747572652d63 # "temperature-c"
0a # unsigned integer(value:10,"if")
66 # text string(6 bytes)
73656e736f72 # "sensor"
a3 # map(number of pairs of data items:3)
01 # unsigned integer(value:1,"href")
6e # text string(14 bytes)
2f73656e736f72732f6c69676874 # "/sensors/light"
09 # unsigned integer(value:9,"rt")
69 # text string(9 bytes)
6c696768742d6c7578 # "light-lux"
0a # unsigned integer(value:10,"if")
66 # text string(6 bytes)
73656e736f72 # "sensor"
a3 # map(number of pairs of data items:3)
01 # unsigned integer(value:1,"href")
78 23 # text string(35 bytes)
687474703a2f2f7777772e6578616d706c652e636f6d2f73656e736f72732f74313233
# "http://www.example.com/sensors/t123"
03 # unsigned integer(value:3,"anchor")
6d # text string(13 bytes)
2f73656e736f72732f74656d70 # "/sensors/temp"
02 # unsigned integer(value:2,"rel")
6b # text string(11 bytes)
6465736372696265646279 # "describedby"
a3 # map(number of pairs of data items:3)
01 # unsigned integer(value:1,"href")
62 # text string(12 bytes)
2f74 # "/t"
03 # unsigned integer(value:3,"anchor")
6d # text string(13 bytes)
2f73656e736f72732f74656d70 # "/sensors/temp"
02 # unsigned integer(value:2,"rel")
69 # text string(9 bytes)
616c7465726e617465 # "alternate"
Figure 4: Links Encoded in CBOR
This specification registers the following additional Internet Media Types:
Type name: application
Subtype name: link-format+cbor
Required parameters: None
Optional parameters: None
Encoding considerations: Resources that use the "application/ link-format+cbor" media type are required to conform to the "application/cbor" Media Type and are therefore subject to the same encoding considerations specified in [RFC7159], Section 6.
Security considerations: As defined in this specification
Published specification: This specification.
Applications that use this media type: None currently known.
Additional information:
--Magic number(s): N/A
--File extension(s): N/A
--Macintosh file type code(s): TEXT
Person & email address to contact for further information: Kepeng Li <likepeng@huawei.com>
Intended usage: COMMON
Change controller: IESG
The security considerations of [RFC6690] and [RFC7049] apply.
TBD.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, October 2010. |
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012. |
| [RFC7049] | Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, October 2013. |
| [I-D.ietf-core-links-json] | Bormann, C., "Representing CoRE Link Collections in JSON", Internet-Draft draft-ietf-core-links-json-00, June 2013. |